1. Field of the Invention
This invention relates to a relief and make-up valve assembly, and more particularly to a dual function valve assembly provided in a fluid pressure device having first and second passages, such that when a preselected high pressure is reached in the first passage then the first passage is brought into open communication with the second passage so as to allow a fluid to flow from the former to the latter, thereby relieving the pressure therefrom, and when the pressure in the first passage becomes lower than that in the second passage, then the second passage is brought into open communication with the first passage, thereby supplying a fluid from the former to the latter to make up or supplement the pressure level in the first passage.
2. Description of the Prior Art
A valve assembly is known, in which, as disclosed in Japanese Pat. Publication No. Sho 38-13336, issued to F. H. Tennis and published July 26, 1963, a pilot-operated relief valve and a make-up valve are combined with each other. However, valve assemblies of this type suffer several disadvantages. Firstly, they are extremely complex and their manufacturing cost is high. Secondly, there is a problem of a spring causing binding to a sliding member due to the streams of fluid acting thereon and an inner poppet failing to be properly seated upon restoring a pressure in a high pressure fluid passage to a normal level. Thirdly, since the relief valve built therein is of the pilot-operated type, there is a lag in response upon relieving an excessively high pressure therefrom. Fourthly, the spring urging the outer poppet valve body against its valve seat for fluid make-up is commonly used for biasing an inner side poppet valve body against its valve seat, limitations are imposed on the construction parameters of the spring, thus leading to an undesirable response to a lowered pressure during a make-up operation.
Another valve assembly is known, in which, as disclosed in Japanese Pat. Publication No. Sho 39-26248, issued to F. H. Tennis, et al and published Nov. 18, 1964, a direct-acting type relief valve and a make-up valve are combined with each other. However, such valve assembly is also undesirably complex, and the manufacturing cost is high. For example, separate high and low pressure poppets and separate valve seats are required. Moreover, the high pressure poppet often becomes unstable during the period between initially opening and the fully opened position. In addition, a fluid pressure device equipped with such a valve assembly can not effectively be operated in a range down to the relief pressure. Still further, the high pressure relief opening is small in diameter, as compared with the size of the entire valve assembly, so that its pressure-relieving capacity is too small.